Method and apparatus providing wave division multiplexing optical communication system with active carrier hopping

ABSTRACT

A wave division multiplexing (WDM) system is disclosed which accommodates shifts in the resonant frequency of optical modulators by using at least two carriers per optical communications channel and at least two resonant modulator circuits respectively associated with the carriers within each optical modulator. A first resonant modulator circuit resonates with a first carrier and a second resonates with a second carrier when there is a shift in resonance frequency of the at least two resonant optical modulator circuits. A switch circuit controls which carrier is being modulated by its respective resonant modulator circuit.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 13/547,139, filed Jul. 12, 2012, which is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

Embodiments of the present invention relate to wave division multiplexing optical communication systems which compensate for operational changes in photonic devices due to temperature changes.

BACKGROUND OF THE INVENTION

Silicon-based integrated circuits have long been used as a platform for microelectronic applications. More recently, as speed, bandwidth and signal processing requirements have increased, optical systems are now also being integrated on silicon-based integrated circuits.

Thus, instead of or in addition to using silicon to facilitate the flow of electricity, silicon is used to direct the flow of light. While the speed of electricity and the speed of light are the same, light is able to carry more data over a given optical path than can electricity over a given electrical path. Accordingly, there are significant advantages to using light as a data carrier. Furthermore, using silicon as the optical medium allows for application of, and tight integration with, existing silicon integrated circuit technologies. Silicon is transparent to infrared light with wavelengths above about 1.1 micrometers. For telecommunications wavelengths, silicon has a refractive index of about 3.45, whereas silicon dioxide has a refractive index of about 1.44. The tight optical confinement provided by this high index contrast allows for microscopic optical waveguides, which may have cross-sectional dimensions of only a few hundred nanometers, thus facilitating integration with current semiconductor technologies. In addition, silicon photonic devices can be made using existing semiconductor fabrication techniques used for CMOS circuits, and because silicon is already used as the substrate for most integrated circuits, it is possible to create hybrid devices in which the optical and electronic components are integrated onto a single microchip.

In practice, silicon photonics can be implemented using silicon-on-insulator (SOI) technology or bulk silicon technology. In either case, in order for the silicon photonic components, such as waveguides, to remain optically independent from the underlying silicon of the wafer on which they are fabricated, it is necessary to have an intervening dielectric material. This is usually a dielectric such as silica (silicon dioxide), which has a much lower refractive, index (about 1.44) than silicon in the wavelength region of interest. Silica is also used on the top and sides of a silicon waveguide core, thus forming a cladding around the entire waveguide core. This results in total internal reflection of light at the silicon core-silica cladding interface and thus transmitted light remains in the silicon waveguide core.

One communications technique which can communicate large amounts of data over a waveguide optical link is known as wave division multiplexing (WDM). A typical example of data propagation in a WDM system is illustrated in FIG. 1. As shown, an optical transmission system 100 includes, for example, a plurality of silicon waveguides 110 a . . . 110 n, collectively shown as waveguides 110, each carrying data of an optical communications channel. The system 100 includes multiple data input channels 120 a . . . 120 n, collectively shown as 120, where each data input channel 120 a . . . 120 n transmits data in the form of pulses of light or as electrical signals. In order to simultaneously transmit the data carried on the multiple data input channels 120 a . . . 120 n, the data in each data input channel 120 a . . . 120 n is modulated onto a respective optical carrier having a wavelength λ₁ . . . λ_(n), by a respective resonant optical modulator 130 a . . . 130 n. The outputs of the modulators 130 a . . .130 n form respective optical communicating channels. The resonant optical modulators 130 a . . . 130 n are collectively shown as 130. The optical carriers at wavelengths λ₁ . . . λ_(n), can be supplied to each resonant optical modulator 130 a . . . 130 n by a highly accurate temperature controlled laser source 136. The modulated light output from each resonant optical modulator 130 a . . . 130 n is provided to a respective waveguide 110 a . . . 110 n and the outputs from the waveguides 110 are then multiplexed into a single optical transmission channel waveguide 150 by an optical multiplexer 140. The multiplexed light is then transmitted along the waveguide 150 to an endpoint (not shown) where the data modulated light is de-multiplexed and demodulated before being used by an endpoint device.

The resonant optical modulators 130 a . . . 130 n, which may be ring modulators, are designed to resonate at their respective carrier wavelengths λ₁ . . . λ_(n). The resonant optical modulators 130 a . . . 130 n have resonant cavities and are constructed of materials with refractive indices, both of which are affected by temperature changes. The changes in temperature of the resonant optical modulators 130 a . . . 130 n cause their respective resonant frequencies to change and move away from their respective carrier wavelengths λ₁ . . . λ_(n). As a result, the modulation index of the modulators 130 a . . . 130 n drops resulting in reduced signal-to-noise ratio and the potential for data transmission errors. Therefore, there is a need for a WDM optical communications system which can adapt to temperature or other changes which might adversely affect the modulation of data signals onto an optical communication channel by a resonant optical modulator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of a conventional a wave division multiplex transmission system;

FIG. 2 shows a wave division multiplexing and demultiplexing system in accordance with an example embodiment of the invention.

FIG. 3A, shows an example of a conventional WDM optical communications system channel structure;

FIG. 3B shows an example of a WDM optical communications system channel structure which may be used with the FIG. 2 embodiment;

FIG. 4 illustrates one example of how a temperature change affects modulators of the FIG. 2 embodiment;

FIG. 5 illustrates an example of a temperature detector which detects changes in resonance frequency and which can be used with the FIG. 2 embodiment;

FIG. 6A illustrates an example of another detector which detects changes in resonance frequency of a modulator and which can be used with the FIG. 2 embodiment;

FIG. 6B illustrates an eye diagram illustrating operation of the FIG. 6A detector;

FIGS. 7A, 7B and 7C show respective example embodiments of alternative demodulators which may be used in the FIG. 2 embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Example embodiments described herein provide a wave division multiplex (WDM) optical communication system having a plurality of optical communications channels which can be multiplexed and demultiplexed. The WDM optical communications system can be partially or entirely integrated onto a die. Each optical communication channel has at least two spaced carriers and a data modulator having at least a first and a second resonant optical modulator circuit, each associated with a respective carrier. The first resonant optical modulator circuit has a resonant frequency at its respective carrier wavelength and the second resonant modulator circuit has a resonant frequency which is offset from its respective carrier wavelength. When temperature or other changes cause the resonance of the first resonant modulator circuit to move in a direction out of resonance with its associated carrier wavelength the same temperature or other change will also cause the resonant frequency of the second modulator circuit to move in a direction towards resonance with its associated carrier wavelength. A detector which detects when the first modulator circuit is sufficiently out of resonance with its respective carrier wavelength and the second modulator circuit is in sufficient resonance with its respective carrier wavelength is used to control switching of data modulation from the first to the second modulator circuit. As a consequence, data for a given optical communication channel is switched to a different channel carrier wavelength where it is reliably modulated by the second modulator circuit.

Although the embodiments described and illustrated herein employ two modulator circuits and associated carrier wavelengths per optical communications channel, this is merely an example as more than two resonant modulator circuits, each having a respective carrier wavelength can be used for each optical communications channel. Also, although embodiments are described with reference to a temperature change causing changes in the resonant frequency of modulator circuits, the embodiments are not limited to being responsive to only temperature changes. The embodiments can also be used in any environment where there is an induced change in the resonant frequency of modulator circuits.

The switch from the first resonant optical modulator circuit and respective carrier wavelength to the second resonant optical modulator circuit and respective carrier wavelength for transmitting data on an optical communication channel on the multiplex side can be communicated through an existing optical communications channel, or a separate electrical communications channel, to a demultiplexing side of the system. The demultiplexing side of the system uses the switching information to switch to the new modulated carrier wavelength and to an associated demodulator for proper data demodulation for each optical communications channel. The switching information is transmitted to the demultiplexing side prior to switching from the first to the second resonant optical modulator circuit on the multiplexing side so the demodulators can switch in synchronism with the switching of the modulator circuits.

FIG. 2 illustrates the multiplexing 10 and demultiplexing 12 sides of a WDM system in an example embodiment. The multiplex side 10 includes a laser source 101 providing at least two carrier wavelengths to each resonant optical modulator 170 a . . . 170 n. Each of the resonant optical modulators 170 a . . . 170 n contain at least two distinct first and second optical modulator circuits. For resonant optical modulator 170 a, these are optical modulator circuits 104, 104 a; for optical modulator circuit 170 b, these are optical modulator circuits 108, 108 a; for resonant optical modulator 170 n, these are optical modulator circuits 113, 113 a. Each resonant optical modulator 170 a . . . 170 n output is a respective optical communications channel which is supplied to respective waveguides 110 a . . . 110 n. Each optical modulator 170 a . . . 170 n also receives respective data input as DATA 1, DATA2 . . . DATAN which can be supplied to each of the first and second optical modulator circuits within a given modulator 170 a . . . 170 n. The outputs of the resonant optical modulators 170 a . . . 170 n are supplied via respective waveguides 110 a . . . 110 n to an optical multiplexer 140 which multiplexes the optical communication channels on waveguides 110 onto a waveguide 142 which sends the multiplexed optical communications channels to the demultiplexing side 12 of the system.

The demultiplexing side 12 includes optical demultiplexer 147 for demultiplexing the signals on waveguide 142, a plurality of waveguides 172 a . . . 172 n for the respective ones of demultiplexed optical communications channels, and a plurality of demodulation circuits 109 a . . . 109 n for respectively demodulating the received carrier on waveguides 172 a . . . 172 n to supply demodulated data as DATA 1 (OUT) . . . DATAN (OUT).

Before describing the operation of the FIG. 2 system in greater detail reference is made to FIGS. 3A and 3B. FIG. 3A shows an example of an optical communications channel carrier spacing in a conventional WDM system. Each of the optical communications channels are illustrated as having respective spaced wavelengths 115 of λ₁, λ₂, . . . λ_(n) which can be modulated. In the embodiment illustrated in FIG. 2 at least one additional carrier 117 is added for each optical communications channel. The additional carriers are offset from respective FIG. 3A carrier wavelengths and are shown in FIG. 3B as wavelengths λ₁′, λ₂′ . . . λ_(n)′. Thus, at least two carriers 115, 117 are available for data modulation for each optical communications channel on the multiplexing side 10 of the FIG. 2 system.

Referring back to FIG. 2 laser source 101 is temperature controlled to provide stable carrier wavelengths and may be formed of one or more temperature controlled lasers. Laser source 101 provides the at least two carrier wavelengths shown in FIG. 3B, for each optical communications channel. For example, the optical communications channel which is associated with modulator 170 a has two carriers at respective wavelengths λ₁ and λ₁′ provided to modulator 170 a. The same is true for each of the other optical communications channels associated with modulators 170 b . . . 170 n. It should be noted that although laser source 101 is shown as having a respective waveguide connection to each of the modulators 170 a . . . 170 n there may in practice be a single output from laser source 101 at all carrier wavelengths which is fed to all the modulators 170 a . . . 170 n. A modulator 170 a . . . 170 n will only resonant with and modulate data onto a respective one of the at least two carrier wavelengths of its associated optical communications channel.

As noted, each modulator 170 a . . . 170 n includes at least a first and second ring resonator optical modulation circuit, which are designed to resonate with only one of the two carriers provided for its associated optical communications channel. Thus, modulator 170 a contains optical modulation circuits 104 and 104 a each designed to be resonant with one of the two supplied carriers, e.g. λ₁ (for 104) and λ₁′ (for 104 a). Likewise modulator 170 b contains resonant circuits 108, 108 a associated with carriers λ₂ (for 108) and λ₂′ (for 108 a), and modulator 170 n contains resonant circuits 113, 113 a respectively associated with carriers λ_(n), (for 113) and λ_(n)′ (for 113 a). The resonant frequencies of the two modulator circuits in each of modulators 170 a . . . 170 n are such that when a first of the two resonant circuits, e.g. 104, is resonating with its respective carrier at a given temperature, e.g., λ₁ the other resonant circuit 104 a is not resonant with its respective carrier λ₁′ because the resonant frequency of the second modulator circuit 104 a is shifted away from its associated carrier λ₄₀ ′.

The relationship of the carriers to the modulation circuits in each of modulator 170 a . . . 170 n and the effect of temperature changes on each is now explained with reference to FIG. 4, using the modulator circuits 104 and 104 a of modulator 170 a as an example. The FIG. 4 diagram shows an arrangement which exists at ambient temperature, denoted as T=To. In this case, ring modulator circuit 104 has a resonant frequency λ_(1a) which aligns well with the channel carrier wavelength λ₁. Accordingly, ring modulator circuit 104 can reliably transmit data denoted as DATA1 onto the carrier wavelength λ₁. However, the resonant frequency λ₂a of the ring modulator 104 a is not aligned with the wavelength of its associated carrier λ₁′ and therefore cannot reliably modulate the data DATA1 onto carrier λ₁′. Moreover, each of the ring modulators 104, 104 a has a respective driver circuit 106, 106 a (FIG. 2) which is selectively enabled by a switch command on control line 111, such that only one ring modulator 104, 104 a is active at a time. At ambient temperature T=To, only ring modulator circuit 104 is active while modulator 104 a is inactive.

When the temperature of the ring modulator circuits 104, 104 a increase to T=To+ΔT, the resonant frequency of the ring modulator 104 shifts to a resonant frequency at a wavelength λ_(1b). At this resonant frequency modulator 104 can still resonate with its respective carrier a λ₁, but begins to lose its ability to resonate with the carrier However, this same temperature change also shifts the resonant frequency λ_(2a) of ring modulator 104 a to the point where it aligns with and can now resonate with its respective carrier at λ₁′. So at a temperature of T=To+ΔT both modulators 104 and 104 a are capable of modulating their respective carriers λ₁ and λ₁′ with data.

If the temperature of modulator circuits 104 and 104 a further increases to T=To+2ΔT, the first ring modulator circuit 104 loses its ability to reliably modulate data on its respective carrier λ₁ while the second ring modulator 104 a is still able to resonate with and reliably modulate data onto its carrier λ₁′.

The bottom chart on FIG. 4 summarizes the operational states of ring modulators 104, 104 a with respect to their ability to resonate with and modulate data on their respective carriers λ₁ and λ₁′ under the various temperature conditions. At a predetermined temperature between T=To, and T=To+2ΔT, the ring modulator 104 will be turned off and ring modulator 104 a turned on so that only the latter now modulates incoming data (DATA1) onto its respective carrier λ₁′.

The switch point is a predetermined temperature where a first 104 of the two modulator circuits 104, 104 a is instructed to stop modulating its respective carrier λ₁ with data and the second modulator circuit 104 a is instructed to start modulating its respective carrier λ₁′ with data. This predetermined temperature switch point may be applied globally such that when the switch point temperature is detected all of the ring modulators 170 a . . . 170 n each will simultaneously shift from their first to their second modulator circuit. Since modulators 170 a . . . 170 n can be provided on a common integrated circuit die, a die temperature detector can be commonly used to determine a switch point for all modulators 170 a . . . 170 n. The switch command is commonly applied to all modulators 170 a . . . 170 n by a control line 111.

FIG. 5 illustrates a temperature detector circuit which can be used to determine when the switch point is reached and issue a corresponding switch command on control line 111 (FIG. 2) to modulators 170 a . . . 170 n. It includes a temperature sensor 133, such as a thermistor, which has an output connected to one input of a comparator 134. Another input to comparator 134 provides a threshold reference value which must be exceeded for comparator 134 to switch the value of an output signal. This reference value may be selected to correspond to the predetermined temperature which must be reached for the switch command to be issued. FIG. 5 also shows a hysteresis circuit 127 in the form of a Schmitt Trigger which provides the switch command signal on control line 111 to control a shift from the first ring modulator circuit to the second for all of the modulators 170 a . . . 170 n.

Referring again to FIG. 2, each of the ring modulator circuits within a modulator 170 a . . . 170 n includes an associated driver circuit 106, 106 a; 107, 107 a; 115, 115 a, for driving the first and second modulator circuit of each modulator 170 a . . . 170 n with respective input data, e.g. DATA1, . . . DATAN. The driver circuits respond to the switch command signal from the FIG. 5 circuit on control line 111 to control the switch from the first modulator circuit to the second for the modulators 170 a . . . 170 n. For example, the driver circuits 106, 106 a control the switch from first modulator circuit 104 to the second modulator circuit 104 a, for the modulator 170 a. The driver circuits 106, 106 a have an enable/disable input line which receives the switch command from control line 111. The driver circuits, e.g. 106, 106 a, can be electrical circuits receiving electrical data input or optical circuits receiving optical data input. For a system using two internal modulation circuits for each of modulators 170 a . . . 170 n, a logic value of “1” or “0” can be supplied by Schmitt Trigger 127 on control line 111 as the switch command.

The switch command also needs to be communicated to the demodulation circuits on the demultiplexing side 12 of the FIG. 2 system so that demodulators which are demodulating a particular carrier, e.g. λ₁, λ₁′ for an optical communications channel know which channel carrier to demodulate. The switch command can be sent on a particular optical data channel on the multiplexing side 10 to the demultiplexing side 12. For example, the FIG. 2 embodiment shows the modulator 170 n being used for this purpose as the wavelength switch command can be part of the input DATAN, although any optical data channel can be used. Alternatively, a separate electrical channel can be used to transmit the switch command from the multiplexing side 10 to the demultiplexing side 12. The switch command is sent before an actual switch from the first to the second modulator circuit within the modulators 170 a . . . 170 n occurs on the multiplexing side 10 so the demodulators know when to switch and can switch in synchronism with the modulators 170 a . . . 170 n.

FIGS. 6A and 6B show another arrangement for providing a switch command signal on control line 111. FIG. 6A shows an alternative modulator 170 a′ having respective modulator circuits 104, 104 a and associated driver circuits 106, 106 a. A light tap 124, 124 a can be provided in association with the output of a respective modulator circuit 104, 104 a to receive a small portion of modulated light representing the light output by the respective modulator circuits 104, 104 a. The light taps 124, 124 a connect with respective photo detectors 125, 125 a. The outputs of the photo detectors 125, 125 a are fed to respective comparators 126, 126 a which have signal thresholds representing a particular level of signal power delivered by the photo detector 125, 125 a. When the detected power for modulator circuit 104 falls below a threshold value set by the comparator 126 this indicates that resonance of resonance circuit 104 with its respective carrier λ₁ is being lost. FIG. 6B shows an eye diagram representing the modulation index of a modulation circuit. When the resonant circuit 104 is in complete resonance with its associated carrier λ₁ a high modulation index represented by power value 129 is evident. However, when the resonant circuit 104 starts to become out of resonance with its associated carrier λ₁ power is reduced and the modulation index drops according to the amount of shift in the resonance frequency of the modulator circuit 104 relative to the carrier wavelength λ₁. This lowered modulation index is represented by the power value 131 in FIG. 6B. Comparator 125 can be used to determine when the lowered modulation index is achieved, for example, a modulation index lowered by 8 dB, and provide a switch signal indicating that a switch should occur from the first modulation circuit 104 to the second 104 a. A hysteresis circuit 127, such as a Schmitt Trigger, provides the switch control signal.

FIG. 6A also shows a separate detector, which includes tap 124 a, photo detector 125 a, comparator 126 a, a Schmitt Trigger and hysteresis circuit 127 a, for detecting loss of resonance of the second modulator circuit 104 a′ with its respective carrier λ₁′. This is used to determine when the temperature of the modulators 170 a . . . 170 n has lowered to the point where a switch should occur from the second modulator circuit 104 a modulating its carrier λ₁′ back to the first modulator circuit 104 for modulating its carrier λ₁. The FIG. 5 temperature detector output can also be used to switch back from the second 104 a to the first 104 modulator circuit when the detected temperature falls below the switch threshold set by comparator 134.

Referring back to FIG. 2, the carriers of optical communications channels are demultiplexed by demultiplexer 147 and the demultiplexed optical communications channels are provided on respective waveguides 172 a . . . 172 n. Each of the demultiplexed optical communications channels can have data modulated on one of at least two carriers of the communications channel, e.g. λ₁, or λ₁′ for the optical communications channel associated with modulator 170 a on the multiplex side 10. Each optical communications channel on respective waveguides 172 a . . . 172 n has an associated demodulator 109 a . . . 109 n, each of which contains at least a first and second wide band resonant ring demodulator circuit. For example, demodulator 109 a includes first and second resonant ring demodulator circuits 200, 200 a for respectively demodulating the carriers λ₁, λ₁′. Likewise demodulators 109 b and 109 n have demodulator circuits 201, 201 a and 205, 205 a respectively. Each of the demodulator circuits, 200, 200 a; 201, 201 a; 205, 205 a are wide band resonant ring demodulator circuits which are capable of resonating with and demodulating a respective carrier over wide ranges of temperature changes, which otherwise would adversely affect a modulating circuit, e.g. 104, 104 a.

The switch command generated on the multiplexer side 10 and sent to the demultiplexer side 12 tells the demodulators 109 a . . . 109 n which internal demodulation circuit to use corresponding to the carrier wavelength which is being modulated for the corresponding optical communications channel. Before a shift from a first demodulator circuit, e.g. 200 to second, e.g. 200 a, occurs within a demodulator 109 a . . . 109 n the switch command is received, demodulated off the communications channel communicating it, e.g. demodulator 109 n, and sent on control line 113 to all of the demodulators 109 a . . . 109 n instructing a switch from the first internal demodulator circuit, e.g. 200 to the second, e.g. 200 a. Each demodulator circuit, e.g. 200, 200 a has an associated light tap for receiving demodulated data and passing it to a respective photo detector e.g. 123, 123 a for demodulator 109 a, 223, 223 a for demodulator 109 b, and 323, 323 a for demodulator 109 n. The respective photo detectors 123, 123 a, 223, 223 a, 323, 323 a can be turned on or off by the received switch command on control line 113 such that only one photo detector of each pair is on at a time.

For a system using two modulator circuits per modulator 170 a . . . 170 n, one logic state of a switch command e.g. “1” can be used to turn on a first photo detector, e.g. 123 while turning off a second photo detector e.g. 123 a. Likewise, the other logic state, e.g. “0”, turns off the first photo detector 123 and turns on the second photo detector 123 a. Each of the demodulators 109 a . . . 109 n has the output of the photo detector e.g. 123, 123 a combined to provide a data output signal, e.g. DATA 1 (Out) for demodulator 109 a. In this way, one of the demodulator circuits e.g. 200, 200 a, which demodulates its associated channel carrier λ₁, λ₁′, is selected for providing a data output signal e.g. DATA 1 (Out) for its associated optical communications channel.

FIG. 7A illustrates a modified demodulator 109′ which can be used in the FIG. 2 system in place of the demodulators 109 a . . . 109 n. Here, instead of combining the outputs of photo detectors 135 and 135 a, the photo detector outputs remain separate. Here again, the switch command on line 113 is used to select one or the other of photo detectors 135, 135 a for output.

FIG. 7B illustrates another demodulator 109″ which can be used in the FIG. 2 system in place of the demodulators 109 a . . . 109 n. Here each demodulator circuit within demodulator 109″ is formed by pair of narrow band ring demodulators 220, 222 and 220 a, 222 a. The first pair of ring demodulators, e.g. 220, 222 have resonant frequencies slightly offset from one another, with one being centered on the first carrier of a optical communications channel and the other offset from that. With the FIG. 7B arrangement if a temperature change is enough to shift resonance of the narrow band ring demodulator 220 out of resonance with carrier λ₁, but the temperature change is not yet sufficient to cause the modulators 170 a . . . 170 n to switch to the carrier λ₁′, demodulator circuit 222 moves into closer resonance with the first carrier λ₁. In this case, the output of both ring demodulators 220 and 222 are optically combined and fed to a single photo detector 137 which provides a data output signal, e.g. DATA1(OUT).

A second demodulation circuit is also included within demodulator 109″ and is formed by a second pair of narrow band ring demodulators 220 a, 222 a. They are arranged such that one of the ring demodulators 220 a has a resonant frequency which is centered on the second carrier wavelength of a communication channel, e.g. λ₁′ for the optical communications channel associated with modulator 170 a and with the other ring demodulator 220 b slightly offset from that. Thus if the optical communications channel is using the second carrier λ₁′, and there is temperature change which moves resonance of demodulator circuit 220 a away from the carrier λ₁′, the demodulator circuit 222 a moves into resonance with carrier λ₁′, the outputs of both ring demodulators 220 a, 222 a are combined and sent to a photo detector 137 a. The received switch command on line 113 is used to select one of the photo detectors 137, 137 a output for providing output data, e.g. DATA 1 (Out).

FIG. 7C shows another demodulator 109″' which may be used in the FIG. 2 system. Demodulator 109″′, is similar to demodulator 109″. However, unlike in demodulator 109″, where the optical outputs of the paired ring demodulators 220, 222 and 220 a, 222 a are first optically combined and then sent to respective photodiodes 137, 137 a, in demodulator 109″′ the optical signals from the paired narrow band ring demodulators 220, 222 and 220 a, 222 a are sent directly to the respective photo detectors 137, 137 a.

While various embodiments have been described and illustrated these are merely examples of how the invention may be practiced. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the pending claims. 

What is claimed as new and desired to be protected by Letters Patent of the United States is:
 1. An optical communications system comprising a communications channel having at least two carrier wavelengths which can be modulated with data; and an optical modulator for the communications channel, the optical modulator comprising at least a first and second resonant modulator circuit, each for selectively modulating one of the carrier wavelengths associated with the optical communications channel with data, the first resonant modulator circuit having a resonant frequency at a first of the carrier wavelengths of the optical communications channel and the second resonant modulator circuit having a resonant frequency shifted from and out of resonance with a second of the carrier wavelengths of the optical communications channel.
 2. A system as in claim 1, wherein the resonant frequency of the first and second resonant modulator circuits can shift with a temperature change and the shift in resonant frequency with a temperature change causes the first resonant modulator circuit to move in a direction away from resonance with the first carrier wavelength and the second reasonant modulator in a direction towards obtaining resonance with the second of the carrier wavelengths.
 3. A system as in claim 1, wherein each resonant modulator circuit receives the same input data.
 4. A system as in claim 2, further comprising a first switch circuit for switching from the first to the second modulator circuit in response to a switch command.
 5. A system as in claim 3, further comprising a detector circuit for issuing the switch command when a particular temperature is detected.
 6. A system as in claim 3, further comprising a detector circuit for issuing the switch command when a predetermined amount of shift in the resonance frequency of a first modulator circuit is detected relative to the first carrier wavelength.
 7. A system as in claim 6, wherein the detector circuit detects a drop in a modulation index of the first modulator circuit.
 8. A system as in claim 5, wherein the detector circuit detects the temperature of a die on which the optical modulator is located.
 9. A system as in claim 5, wherein the detector circuit Comprises a temperature sensor which provides an output to a hysteresis circuit.
 10. A system as in claim 9, wherein the hysteresis circuit comprises a Schmitt Trigger.
 11. A system as in claim 7, wherein the detector circuit further comprises a hysteresis circuit.
 12. A system as in claim 11, wherein the hysteresis circuit comprises a Schmitt Trigger.
 13. A system as in claim 4, further comprising an optical demodulator associated with the optical communication channel, the optical demodulator comprising at least two optical demodulator circuits, a first for demodulating data signals on the first carrier wavelength associated with the optical communication channel and a second for demodulating optical signals on the second carrier wavelength associated with the optical communication channel.
 14. A system as in claim 13, wherein the optical demodulator circuits are capable of resonating with and demodulating a respective carrier wavelength over respective changes which otherwise adversely affect a resonant modulator circuit.
 15. A system as in claim 13, further comprising a second switch circuit associated with each optical demodulator for selecting one of the first and second optical demodulator circuits for providing demodulated output data.
 16. A system as in claim 15, wherein the second switch circuit is responsive to a received switch command.
 17. A system as in claim 16, wherein the received switch command is received over an optical communication channel.
 18. A system as in claim 16, wherein the switch command is received over an electrical communication channel.
 19. A system as in claim 13, wherein each optical demodulator circuit has an associated optical tap for providing an optical output signal to a respective photo detector.
 20. A system as in claim 13, wherein each of the said first and second optical demodulator circuits have optical resonant frequencies which are shifted from each other.
 21. An optical communications system, comprising: an optical communications channel having at least two associated carrier wavelengths, at least one of which is transmitting data; an optical demodulator coupled to the optical communications channel, the optical demodulator comprising at least two optical demodulator circuits, a first for demodulating data signals on a first carrier wavelength associated with the optical communication channel and a second for demodulating optical signals on a second carrier wavelength associated with the optical communication channel; and a switch circuit associated with the optical demodulator for selecting one of the first and second optical demodulator circuits corresponding to the carrier wavelength transmitting data for providing demodulated output data.
 22. An optical communication system as in claim 21, wherein the switch circuit is responsive to a received switch command.
 23. An optical communication system as in claim 22, wherein the received switch command is received over one of an optical and electrical communications channel.
 24. An optical communication system as in claim 22, wherein the received switch command is received from a circuit which controls which of the carrier wavelengths contains modulated data.
 25. An optical communication method comprising: receiving an optical signal over an optical communications channel comprising at least two carrier wavelengths, one of which contains data; demodulating the data using one of at least two demodulator circuits each of which is capable of resonating with and demodulating data from a respective one of the carrier wavelengths; and selecting which of the demodulator circuits will be used to demodulate the data in accordance with which of the at least two carrier wavelengths contains data.
 26. An optical communications system as in claim 25, further comprising receiving a command to select one of the demodulator circuits.
 27. An optical communications system as in claim 26, further comprising receiving the select command from a circuit which controls which of the carrier wavelengths contains data. 